iio: core: fix uninitialized data in debugfs

iio: core: fix uninitialized data in debugfs

If *ppos is non-zero then simple_write_to_buffer() will not initialize
the start of buf[].  Non zero values for *ppos aren't going to work
anyway.  Test for them at the start of the function and return -EINVAL.

Fixes: 6d5dd486c715 ("iio: core: make use of simple_write_to_buffer()")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Maxwell Doose <m32285159@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>